1. Field of the Invention
This invention relates to novel highly hard material coated articles and more particularly, it is concerned with highly hard articles, for example, cutting tools such as turning tools and cutters, wear resisting tools such as metallic molds and wire drawing dies, wear parts such as bearings and cams, and decorations such as watch frames, coated for the purpose of raising the wear resistance, heat resistance and corrosion resistance with high hardness, high melting point and high chemical stability compounds such as carbides, nitrides, or carbonitrides of titanium, zirconium, hafnium or their alloys, or those in which oxygen is dissolved, with a thickness of 1 to 20 microns.
2. Description of the Prior Art
It is well known to coat a tool or part with titanium carbide or titanium nitride in order to improve the wear resistance, heat resistance and corrosion resistance thereof and this is put to practical use. For the fabrication of the coated article, the chemical vapor deposition method (CVD method) or dipping-in-fused salt method represented by "TD process" is used, but these methods are all coating methods carried out at high temperatures such as about 1000.degree. C., so thermal deformations or structure changes tend to occur in the substrates of tools or parts although the adhesion strength between the coating film and substrate is considerably large due to diffusion. Thus, their use is considerably limited. For example, if the substrate is of a tool steel, it is annealed and if stainless steel, the corrosion resistance is deteriorated. The substrate of a cemented carbide alloy tends to be brittle.
On the other hand, progress of the physical vapor deposition method (PVD method) is remarkable and titanium carbide or titanium nitride can be coated even at low temperatures such as below 500.degree. C. The so-called physical vapor deposition method includes the vapor deposition method, sputtering method, activated reactive vapor deposition method (ARE method), hollow cathode discharge vapor deposition method (HCD method), ion plating method and the like, but above all, the ion plating method wherein a material to be vapor deposited is much energized as positive ion by applying a negative accelerating voltage to a substrate is most preferable for increasing the adhesion strength between the coating film and substrate. This ion plating method can be classified into two kinds of the high pressure type (low vacuum type) in which ionization is carried out by glow discharge in an ambient gas under a pressure of 0.5 to 3 Pa, and the low pressure type (high vacuum type) in which ionization is carried out by an intermediate electrode, electron irradiation or high frequency electric field under a pressure of 0.1 Pa or less. However, the ion plating method of the high pressure type has the disadvantage that because of its high ambient pressure, the coating film of titanium carbide or titanium nitride tends to be weak and no sufficient adhesion strength is obtained since the acceleration of ions is carried out mainly at the cathode dark portion about the base material. In the case of the low pressure type, on the other hand, a coating film of good quality can readily be obtained and it is possible to accelerate sufficiently ions with a substantially uniform potential gradient, resulting in a high adhesion strength, but the film quality and adhesion strength are largely affected by the ionization efficiency and accelerating voltage. The ordinary ion plating method is not so effective for increasing the adhesion strength because the ionization efficiency is generally low and the energy given to a material to be vapor deposited cannot be increased so much even if the accelerating voltage is increased. Consequently, there have been proposed the HCD method and ARE method whereby the adhesion strength is increased by heating a substrate without applying an accelerating voltage, but the adhesion strength is not sufficient at low temperatures and the advantages of the physical vapor deposition method as described above are lost at high temperatures. When applying an accelerating voltage using the ordinary ion plating apparatus with a low ionization efficiency, for example, the current flowing through an accelerating electrode (in general, substrate) is very small, i.e. about 0.01 to 0.2 A even if the accelerating voltage is considerably high.
The inventors have hitherto employed such an ion plating process in a study on the coating of cemented carbide inserts with a layer of TiN or TiC for comparison with the chemical vapor deposition process, as disclosed in "Thin Solid Films" 54 (1978) p 67-74. According to this study, it is apparent that the wear resistance of carbides coated by ion plating is similar to that of carbides coated by chemical vapor deposition, but the toughness obtained by ion plating is superior to that of chemically vapor deposited coatings. In addition, the inventors have also proposed a coated tool steel comprising a tool steel substrate and a coated film thereon consisting of at least one of titanium, zirconium and hafnium carbides, nitrides and carbonitrides, which has a greater half value width, as disclosed in U.S. Pat. No. 4,169,913. However, these coated articles are not satisfactory in adhesion strength between the substrate and coating.